Currently, and in the recent past, numerous types of sampling devices have been advanced and are available for collection of airborne microorganisms (bioaerosols) from both outdoor and indoor environments. Each device has certain advantages and disadvantages, depending on the particular environment in which it is used and the type of sample to be collected. A reference list of recent comparative studies of bioaerosol sampling instruments (particularly for allergen collection) is provided in a recent article by Cage, et al..sup.1, which focuses on an evaluation of four bioaerosol samplers for use in outdoor environment. The samplers the authors compare include a Rotorod, a Kramer-Collins suction trap, an all-glass impinger (AGI-30), and the Spincon high-volume cyclonic liquid impinger. None of these samplers operates on a continuous or semi-continuous basis to provide samples for rapid or short-time analyses which are of considerable interest to the users.
The literature contains numerous other reviews of bioaerosol samplers and collectors. Chatigny.sup.2, for example, provides a rather full listing of commercially available devices most frequently used in sampling microbial aerosols of various types, sizes, and concentrations.
Both the Andersen multi-stage cascade impactor.sup.3,4 and the AGI-30 all-glass impinger.sup.5,6 have been recommended in the literature for use as laboratory standard samplers. The Andersen Viable Sampler is a well-known, six-stage, multi-orifice cascade impactor-type unit designed specifically for collection of airborne, viable bacteria on nutrient-filled Petri dishes. The resultant colonies can then be subjected to standard microbiological analyses.
The Andersen impactor can provide some direct classification of sizes in incoming particles, as the larger particles entering each successive stage cannot follow the air flow in jets directed at each nutrient treated impaction surface. Rather, the larger particles in each case impact on the surfaces, while the smaller ones pass onto the next stage designed to capture particles of smaller absolute size. The principal shortcoming of these (and typically other impactors, as well) is that the incoming particles are not collected in liquid and, therefore, the particles are subjected to considerable shock forces on impact, which may affect their viability. Further, the samples are not well suited to be subjected to subsequent manipulation that is often required to facilitate some of the instrumental bioparticle analysis techniques to be applied in a timely fashion. Two secondary weaknesses are that only relatively low intake air-flow rates (about 28 liters/min or less) are provided and there is no easy means to collect sequential samples to characterize particulates on a time-dependent basis.
The AGI-30 impinger, is an all-glass unit that operates by drawing aerosols through an inlet tube curved to simulate the human nasal passage. This tube forms all air-jet output. In operation, a one-half atmosphere vacuum is drawn across the jet tube so that a choked-flow condition is maintained, with a typical flow rate of about 12.5 l/min. This flow rate has been found to be useful for collecting microbial particles in the respirable size range of about 0.8 to 15 .mu.m. The jet is directed into a collecting medium of water, typically about 20 ml in volume, but it can be smaller to achieve an increased concentration of microorganisms. The very low intake air flow rate can be a drawback in using this sampler, especially if very low concentrations of particulates in the outdoor environment need to be sampled in a short time frame of a few minutes. No known provision currently exists to operate this type of sampler on a continuous or semi-continuous basis to provide a sequence of collected samples to indicate relatively short-term behavior of an ambient outdoor aerosol.
In these samplers--as well as in a wide variety other collectors.sup.7,8,9,10,11 --a primary concern has been to maintain the viability of the collected bioparticles. According to Cox.sup.12, some attempts to develop improved impingers (in particular) that overcome such problems have been made by other workers, including Shipe.sup.11, et al. and May, and these efforts have been at least partially successful. Cox notes (op. cit.) in the case of May's impinger ". . . the design represents a marked advance over more conventional impingers, but unfortunately, its complex construction in glass and ensuing cost represent disadvantages over classical impingers . . . ".
An additional shortcoming of the various impactors and impingers mentioned so far, is that neither continuous nor short-term integrated samples of the collected microorganisms are made readily available for further analysis. This shortcoming has been addressed, however, by the "sequential" and "tape" samplers. These include the Casella "Airborne Bacterial Sampler-MKII" and the New Brunswick "Microbiological Sampler.".sup.13 These devices operate with a fixed narrow slit providing a particle-laden air jet of known velocity, which impacts on a nutrient agar-filled Petri dish that is mounted on a slowly rotating turntable. Following incubation, the exposed dishes permit colony counts to be made and correlated with time and sampling rate. A "Moving Slide" impactor from Meteorology Research, Inc., can deposit two samples from two identical slits onto a moving slide.sup.14 for subsequent time and concentration analysis. These types of samplers show only limited capability to provide sequential samples suitable for timely instrumental analysis.
Errington and Powell.sup.15 developed a cyclone sampler with an air-particle inlet into which was metered a low flow of suitable liquid that formed a thin liquid layer on the inside wall of the cyclone. The impacted particles on the cyclone wall were then carried by the liquid to the bottom of the unit. Typically, a cyclone collector can provide a somewhat milder stress environment on impacting particles than is usually available in a straight jet or virtual impactor systems.sup.16,17. This device is more effective with larger particles than with smaller ones.
Another technique explored and reported in the literature is the use of electrostatic-based samplers. Rather than using mechanical forces to separate the particles from the air stream, electrical forces are used; these electrical forces may provide further help in collecting the particles on some types of collection media. Typically, the charging of the particles to be sampled is done by passing the sample through the drift region of a high-voltage corona discharge. Either negative or positive coronas can be used, and the charges produced in the drift field are correspondingly also negative or positive.sup.18. Additional details of the utilization of coronas in charging aerosol particles to enhance particle collection are given by Ogawa.sup.19, Lippmann.sup.20, and Swift and Lippmann.sup.21.
In the use of electrostatic precipitators for the collection of biological aerosols the effect, must be considered, which ultraviolet radiation, ozone, or nitrogen oxides, produced during corona generation, may have on the viability of the collected biological materials.
Despite the intense activity in the past to advance and develop bioaerosol samplers, a need remains for a bioaerosol collector that incorporates into a single device a number of features to achieve both improved process performance and greater overall capability than is exhibited by any of the instruments previously noted.
One of the problems with prior art devices mentioned above has been the inability to achieve high collection efficiencies of airborne microorganisms whose sizes are smaller than about 2 .mu.m. Thus, the present invention has as a purpose to provide particle collection with a higher aerosol collection efficiency than with prior art devices, especially for submicron particles.
The second problem has been to be able to rapidly sample and process a sufficient volume of ambient air to ensure that a relatively large number of specific microorganisms of interest are collected (and ultimately concentrated) in a liquid to form a representative sample over a short time period of about two minutes or less. Increased sampling air flow in the collector and concentrator will be another purpose of this invention.
A third purpose will be to provide a continuous, uninterrupted sampling process to ensure that a time sequence of two minute samples over a much longer duration, suitably of the order of hours, will be available.
A fourth purpose will be to provide for partial or total separation of the particles collected, in liquid into one or more size classes to facilitate, the identification and analysis of particles of interest from the much larger total number of collected particles, which includes those representative of the natural ambient microorganisms.
A fifth purpose will be to concentrate the particles in each individual particle size range and thereby develop a time series of aliquots for further analysis of any microorganisms that might be present.
Finally, because many of the aerosol collectors tend to be unreliable and complex in their design, therefore expensive and difficult to operate, a sixth purpose will be to provide a collector/concentrator that will overcome these shortcomings.